[evlatests] Correction on sensitivity 'loss' in 8-bit path

[Barry Clark]

But that's what one would expect from the rolloff.  There is a floor of
clipping noise, and there is no loss of sensitivity as you roll off 
until you hit the floor, which is 10 plus db below the plateau.

Rick Perley wrote:
>     I reported yesterday that there was a significant loss in 
> sensitivity at the bottom edge of the 1-GHz-wide IF when in 8-bit mode, 
> attributing this to the well-known ~5-db rolloff. 
>     The loss of sensitivity is real, but my explanation/association is 
> almost certainly wrong. 
> 
>     A more detailed investigation, using UVHGM to plot the noise 
> histograms as a function of channel shows that the spectacular loss in 
> sensitivity is restricted to the bottom 10 channels *only* -- 20 MHz.  
> The well-known bandpass rolloff covers well over 100 MHz.  Within that 
> span, the sensitivity is completely unchanged for at least 70 MHz -- 
> over most of which which the spectral power density is attenuated by 5 
> dB or more. 
> 
>     I think it far more likely that the sensitivity loss at the very 
> edge is due to the the same issue which we've had all along with subband 
> 1 at the lower edge, and is completely unrelated to the bandpass rolloff 
> effect. 
> 
>     Note that this problem of increased noise in the bottom edge of 
> subband 1 is not present with the 3-bit path. 
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